1. Field of the Invention
The present invention relates to a silting prevention control for preventing silting from occurring in a space between a spool of a solenoid valve and a valve body.
2. Description of the Related Art
A pressure oil circulating through a hydraulic circuit often contains pollutants such as debris. To remove such pollutants, the hydraulic circuit is provided with a filter. Large pollutants can be removed by the filter, but small pollutants can not be removed because they pass through the filter. Therefore, the pollutants gradually accumulate in the space between the valve body and the spool which is a moving member of the solenoid valve. The pollutants collected in the space between the valve body and the spool causes adverse effects such as the limitation of the movement of the spool. Such a phenomenon is called silting causing a problem.
It is assumed that a hydraulic actuator such as a hydraulic pump, a hydraulic motor or a hydraulic cylinder operates when the solenoid valve operates.
Silting is caused when the spool in the valve body of the solenoid valve does not operate for a long period and an amount of the pressure oil flowing the space between the valve body and the spool is small. Conversely, pollutants hardly accumulate in the space between the valve body and the spool when the pressure oil is flowing in an amount sufficient to operate the hydraulic actuator or the spool keeps operating. Accordingly, silting does not occur.
Processing to superimpose a dither signal on a control current instruction which is applied to the solenoid valve in order to prevent silting is conventionally performed. For example, Japanese Utility Model Application Laid-Open Print No. 1-116812 indicates means for preventing a silting phenomenon by supplying an instruction electric current having a dither signal of 100 to 400 Hz superimposed on a control current instruction to be applied to the solenoid valve.
Specifically, the dither signal of a constant amplitude is superimposed on the control current instruction and applied to the solenoid valve as shown in FIG. 5. By superimposition of the dither signal, the spool of the solenoid valve vibrates finely. Then, the flow of the pressure oil in the solenoid valve is changed. Accordingly, small pollutants flow without being accumulated in the space between the valve body and the spool, so that the silting is prevented from occurring.
But, the dither signal is generally not applied to the solenoid valve when the instruction current value is zero. It is because an output value of the solenoid valve shall be zero but the dither signal might be output for its amplitude. When a state that the instruction current value is 0 continues for a long time, a state that the dither signal is not applied to the solenoid valve continues for a long time, and a silting prevention processing is not performed for a long time. The hydraulic circuit often has a situation that the instruction current value becomes zero. Therefore, there is a possibility that the silting cannot be eliminated completely by the conventional processing of superimposing a dither signal.
It is an object of the present invention to make it possible to prevent silting without fail even in a situation that a control current instruction is zero.
The present invention is a silting prevention control device comprising a solenoid valve (13) in which a moving member slides with respect to a valve body according to a control current instruction to output a pressure oil according to a moving position of the moving member and a subject equipment to be controlled which operates according to the pressure oil output from the solenoid valve (13), for preventing silting from occurring in a space between the moving member and the valve body, wherein:
a relation between a time range and a current value of a current instruction to the solenoid valve (13) is predetermined within a range that the subject equipment to be controlled does not operate, and
a silting prevention current instruction is given to the solenoid valve (13) according to the predetermined relation.
The invention will be described specifically with reference to FIG. 1 and FIG. 2.
FIG. 1 is a block diagram of a cooling fan drive using the present invention. In FIG. 1, servo piston 5 is a device to be controlled. When the servo piston 5 operates, swash plate 2a of a hydraulic pump 2 is moved to change an amount of the pressure oil discharged from the hydraulic pump 2. The hydraulic pump 2 is driven by engine 1 to discharge the pressure oil. The pressure oil discharged from the hydraulic pump 2 is supplied to hydraulic motor 7. Upon the supply of the pressure oil to the hydraulic motor 7, cooling fan 3 mounted on an output shaft rotates. Radiator 9 is disposed to face the cooling fan 3, and when the cooling fan 3 rotates, heat of a coolant is radiated by the radiator 9. Hydraulic fan controller 20 outputs a current instruction according to an input engine speed, a coolant temperature and a temperature of the hydraulic oil of torque converter 11. The solenoid valve 13 changes a valve position according to the current instruction output from the hydraulic fan controller 20. When the valve position of the solenoid valve 13 is changed, servo piston 5 operates.
FIG. 2 is a diagram showing a relation between instruction electric current (is) for preventing silting, which is given to the solenoid valve 13, and its supply time t. It is a relation determined in a range that the cooling fan 3 does not operate. According to the above relation, the silting prevention current instruction (is) is given to the solenoid valve 13.
Therefore, the spool of the solenoid valve 13 moves to flow the pressure oil between the valve body and the spool in an amount not to operate the cooling fan 3. Accordingly, even if the control current instruction is zero, contaminants between the valve body and the spool can be removed with reliability.
Because it is designed to apply the current instruction (is) capable of preventing the silting according to the predetermined relation, large contaminants can surely be removed as compared with a conventional case of superimposing a dither signal of a uniform size.
Besides, the silting prevention current instruction (is) is applied in a range that the cooling fan 3 does not operate, so that a discharge capacity of the hydraulic pump 2 does not vary depending on a dither signal having a predetermined frequency as it was caused heretofore.
The relation between the current instruction (is) and the supply time t is naturally different in its quantitative value depending on a size of the solenoid valve itself and a device to be controlled.